1. Field of the Invention
The invention relates to a device for producing disperse mixtures by means of ultrasound and to advantageous uses of such a device. The invention relates in particular to a device for producing miniemulsions, i.e. emulsions having an average droplet diameter of less than 1 xcexcm.
2. Discussion of the Background
Emulsions are disperse multiphase systems comprising at least two liquids which are virtually insoluble in one another, and they possess great importance in the plastics industry, especially in the detergents and cleaning products industry, in the production of cosmetic or pharmaceutical products and, in particular, in food technology as well. Since emulsions comprise at least one hydrophilic and one lipophilic liquid, a distinction is madexe2x80x94depending on the nature of the internal, disperse phasexe2x80x94between oil-in-water (O/W) and water-in-oil (W/O) emulsions. The internal or the external phase may itself in turn be a disperse system and may, for example, include particles of solids dispersed in the respective liquid phase. An overall system of this kind is also referred to as a polyphase emulsion. Owing to the interfacial tension which exists between the drops of the internal phase and of the continuous, external phase, emulsions are in general thermodynamically unstable and so over time there is a phase separation which may be induced, for example, by drop sedimentation or coagulation. In order to prevent such separation it is common to add emulsifying auxiliaries, such as emulsifiers, which lower the interfacial tension, or stabilizers, which, for instance, preventxe2x80x94or at least greatly retardxe2x80x94the sedimentation of the droplets, by increasing the viscosity of the continuous, external phase.
When the at least two components of an emulsion are mixed, the initial result is a coarsely disperse crude emulsion. By supplying mechanical energy, the large drops of the crude emulsion are broken up and the desired fine emulsion is formed. The smallest droplet size achievable in the emulsification process depends not only on the respective input of power in the emulsifying machine but is also critically influenced by the nature and concentration of the emulsifying auxiliaries. For example, in order to produce ultrafine emulsions, the new interfaces which are formed mechanically must be occupied very rapidly by the emulsifier in order to prevent coalescence of the small droplets.
The average size of the droplets of the disperse phase can be determined in accordance with the principle of quasielastic dynamic light scattering (for example, as the z-average droplet diameter dz of the unimodal analysis of the autocorrelation function). In the examples of this document, this was done using a Coulter N4 Plus Particle Analyser from Coulter Scientific Instruments.
A wide variety of dispersing machines are employed for producing emulsions. Emulsions of medium to high viscosity are produced principally by means of rotor-stator systems, such as colloid mills or gear-rim dispersing machines. Low-viscosity emulsions have to date been produced principally using high-pressure homogenizers (HPH). In this case the crude emulsion under a pressure of between 100 and 1000 bar is discharged through the approximately 10 to 200 xcexcm high radial gap of a homogenizing nozzle. It is assumed that drop breakup in this case is mainly attributable to the effect of cavitation. One specific design of a high-pressure homogenizer is the microfluidizer, which operates at relatively low pressures of about 100 bar. However, high-pressure homogenizers are not without their disadvantages. Especially when emulsifying polymerizable systems or when producing multiphase emulsions using particles of solids, it is easy for the narrow radial gap to become clogged. The cleaning subsequently required is time-consuming and complex. Moreover, the high pressures entail sealing problems, especially when using media which attack the sealants. A further disadavantage of high-pressure homogenizers is that the drop size and the throughput are coupled with one another. Apparatus of this kind is therefore unsuitable for producing miniemulsions in whose disperse phase it is intended to disperse particles of solids.
It is known, moreover, that ultrasound can be employed to produce emulsions, to mix fluid mixtures thoroughly or to deagglomerate particles.
German Utility Model Application DE-GM 17 73 768 describes a vessel for ultrasonic treatment of a medium. The sonotrode is attached externally to the vessel wall. Consequently, neither a long range cavitation front nor a sufficient input power for the production of miniemulsions can be achieved. A floating reflector is arranged in the prior art vessel so that the vessel cannot be made pressure-sealed.
U.S. Pat. No. 4,444,961 describes a method for producing polymer beads having narrow particle size distribution. A monomer phase is injected through an orifice plate into a continuous phase. A vibratory exciter is used to induce a oscillatory movement of a piston such that laminer monomer jets which are formed at the orifice plate are broken up into separate droplets. Such an arrangement is suitable only to produce droplets having a diameter in the range of 1 mm or more. Otherwise, the openings in the orifice plate would have to be very small, thus increasing the risk of clogging. In addition, oscillatory frequencies in the range of 100 to 1000 Hertz used in the prior art method are too low for producing fine dispersions.
In German Patent Application DE 39 30 052 A1 an acoustic transducer for sono-chemical reactions is described. The transducer, working in a frequency range of 200 MHz to 2 GHz is not suitable for producing emulsions because cavitation as a major action for dispersing droplets is no longer effective at these high frequencies. Further, the flow cell described in this document has disadvantages: Sonic waves are concentrated in a small local working area within the flow cell such that it cannot be assured that the whole medium is uniformally sonicated.
From GB 2 250 930 A a through-flow device for the ultrasonic treatment of liquid media is known. In this device, an axially emitting sonotrode protrudes into the flow cell. The diameter of the sonotrode is small relative to the diameter of the reaction chamber.
A similar arrangement is also described in U.S. Pat. No. 5,108,654.
Moreover, European Patent Application EP 0 584 685 A2 discloses a reactor for implementing chemical reactions, in which at least 9 ultrasonic emitters are arranged on, or integrated into, the wall of a stirred kettle.
The known ultrasonic devices, however, have numerous disadvantages. Although the dimensions of the flow channels and/or reaction chambers mean that it is possible to avoid the risk of clogging known from high-pressure homogenizers, the achievable droplet size cannot be adequately predetermined using the known ultrasonic devices. For example, the Applicant has found that the droplet size increases as the viscosity of the disperse phase goes up. Moreover, a limiting size is reached which is dependent on the specific power input and below which it is impossible to pass even by further lengthening the sonication period. Even this theoretical limiting drop diameter is only achieved if the emulsifying auxiliaries that are used occupy the interface with sufficient rapidity.
In the case of the rodlike sonotrodes that protrude into the reaction chamber, sonication is limited to the area directly surrounding the end of the sonotrode. The major part of the reaction chamber is either not sonicated at all or is sonicated inadequately.
In an ultrasonic device for producing emulsions described in German Patent Application DE 196 12 349 A1, it is foreseen to direct the medium which has to be emulgated via a nozzle onto a sonotrode. While it can be assured that the whole medium is, at least for a short period of time, treated in the ultrasonic field, there is a danger that some fractions of the medium will quickly leave the treatment cell, while other fractions remain quite long under the effect of the sonotrode. A similar arrangement is described in U.S. Pat. No. 5,032,027.
Finally, in FIG. 2 of German Patent Application DE 28 46 462 a flow chamber for producing emulsions by means of ultrasound is shown, wherein said flow chamber is formed by the front faces of two sonotrodes. The slit foreseen between the two sonotrodes is rather narrow (3 to 25 mm) such that the device is less suitable for use in emulsion polymerization on an industrial scale.
A particular disadvantage associated with known devices is that process parameters determined on the laboratory scale cannot be reliably transferred to the industrial scale. This is because the achievable drop diameter depends on numerous parameters, including the frequency and power of the ultrasound, the vibration amplitude, the sonication period, the dynamic viscosity and density of the continuous and of the disperse phase, and the interfacial tension. If, for example, optimum process parameters for a given system are determined on the laboratory scale, these data cannot be transferred directly, with the known devices, to the industrial scale.
It is an object of the present invention to indicate a device which permits the direct transfer of laboratory process parameters to large-scale industrial productions. A further intention of the device of the invention is to make it possible to incorporate even soluble and/or poorly soluble substances into the developing multiphase system in the course of emulsification.
We have found that this object is achieved by a device for producing disperse mixtures by means of ultrasound, having a housing, a reaction chamber within the housing and at least one means of transmitting ultrasonic waves, said means having a free emitting surface which is in effective connection with the reaction chamber, in which device the emitting surface of the means of transmitting ultrasonic waves corresponds essentially to the surface of the reaction chamber and, if the reaction chamber is a subsection of a through-flow reaction channel, extends essentially over the entire width of the channel, and wherein the reaction chamber depth which is essentially vertical with respect to the emitting surface is lower than the maximum effective depth of the ultrasound transmission means.
The device of the invention is accordingly configured so that the entire reaction chamber can be sonicated uniformly with ultrasonic waves. The sonic dead spaces which occur with conventional ultrasound emulsifying devices are largely eliminated with the device of the invention.